The GNOME Display Manager (GDM) can allow users to login without credentials
which can be useful for public kiosk scenarios. Allowing users to login without credentials
or "guest" account access has inherent security risks and should be disabled. To do disable
timed logins or guest account access, set the TimedLoginEnable to false in
the [daemon] section in /etc/gdm/custom.conf. For example:
[daemon]
TimedLoginEnable=false